Fix combination adjustment

[simongray]

Currently, the relative sizes of a combination can be adjusted by dragging the invisible middle bar in the middle. However, the drop event occurs whenever the cursor leaves the area of the combination, e.g. when is placed above or below the element, but also when it is dragged over an embedded PDF. The latter case is the more annoying one since it makes it extremely cumbersome to adjust.

The per-element drag action should be replaced by a page-wide drag&drop detection if possible.